1. Field of the Invention
The invention relates to a novel organic light emitting device (OLED), to novel materials that are capable of phosphorescence, and to methods of making the device and novel materials.
2. Related Technology
Luminescent conjugated polymers are a new technologically important class of materials that will be used in light emitting display devices for the next generation of information technology based consumer products. The principle interest in the use of polymers, as opposed to inorganic semiconducting and organic dye materials, lies in the scope for low-cost manufacturing, using solution-processing of film-forming materials. Since the last decade much effort has been devoted to the improvement of the emission efficiency of OLEDs either by developing highly efficient materials or efficient device structures.
In OLEDs, electrons and holes are injected from opposite electrodes and are combined to form two types of excitons; spin-symmetric triplets and spin-antisymmetric singlets in a theoretical ratio of 3:1. Radiative decay from the singlets is fast (fluorescence), but from the triplets (phosphorescence) it is formally forbidden by the requirement of the spin conservation.
Initially spurred on by this understanding that the maximum internal quantum efficiency of an OLED was limited to 25% the idea of transferring both singlets and triplets to a phosphorescent dopant was conceived. Such a dopant ideally is able to accept both singlet and triplet excitons from the organic material and generate luminescence, particularly electroluminescence from both.
This idea is still highly applicable even given recent studies questioning the 3:1 triplet to singlet ratio predicted by the spin-independent recombination model. Recent studies suggest that the proportion of triplet excitons generated in small molecule devices is indeed close to 75%, (Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R. Phys. Rev. B 1999, 60, 14422) whereas in some electrically excited conjugated polymers it is suggested to be around 50%, (Cao, Y.; Parker, I. D.; Yu, G.; Zhang, C.; Heeger, A. J. Nature 1999, 397, 414 and Wilson, J. S.; Dhoot, A. S.; Seeley, A. J. A. B.; Khan, M. S.; Köhler, A.; Friend, R. H. Nature 2001, 413, 828). Evidence for the copious generation of triplet excitons in polymer LEDs has been obtained by magnetic and optical observations.
Many have studied the incorporation by blending of phosphorescent materials into a semiconductive layer. Blends incorporate a phosphorescent dopant and a small molecule or a non-conjugated polymer host. The host material is required to transport charge to the dopant. Typical examples of good charge transport materials are polymers with extended conjugation lengths. Conjugated polymers have also been disclosed as hosts, for example a blend of Eu(dnm)3phen in CN-PPP with a quantum efficiency of 1.1%. [Adv. Mater., 1999, 11, 1349.]. Similarly, Phys. Rev. B 2001, 63, 235206 discloses poly(9,9-dioctylfluorene) doped with 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum (II). Reference also may be made to WO 03/091355, which discloses a material capable of luminescence comprising a polymer or oligomer; and an organometallic characterized in that the organometallic is covalently bound to the polymer or oligomer and the nature, location and/or proportion of the polymer or oligomer and of the organometallic in the material are selected so that the luminescence predominantly is phosphorescence. It is said that this material generally is superior to a polymer blend incorporating a phosphorescent dopant. This is because problems relating to morphology changes such as aggregation and phase separation are avoided. It is further said that the controlled structure of the material means that the location and mobility of the organometallic in the material is spatially controlled. This spatial control enables control of the interaction between the polymer or oligomer and the organometallic.
However, there are problems with using conjugated polymers as host materials. The host material must have a sufficiently high T1 energy level (the energy level of the lowest triplet excited state) to avoid quenching of the dopant. Put simply, quenching can occur when the T1 level of the host material is lower than the T1 of the dopant so that non-radiative transfer of the triplet exciton from the dopant to the host material is more favourable than radiative decay. This presents a particular problem for blue dopants and green dopants, which have larger band gaps than red dopants, which means higher T1 levels. This is a problem because a typical good host material with good charge transport properties will typically have relatively low T1 level due to extended regions of conjugation, as discussed above. Thus, for blue and green dopants it is especially difficult to find host materials with sufficiently high T1 levels. To date, polyvinylcarbazole has been disclosed as a host material with a sufficiently high T1 level to render it suitable for use with green dopants. However, polyvinylcarbazole has inferior charge transport properties as compared with conjugated polymers, which leads to poor lifetimes when used in a device. Carbazole compounds as hosts for triplet emitters are the subject of J. Am. Chem. Soc. 2004, 126, 7718 to 7727.
Therefore, it will be understood that there exists a need to provide a host material for higher triplet energy materials such as green dopants, which has a sufficiently high T1 level combined with good charge transport properties.
In this regard, a material with good charge transport properties can be characterized by:                a T1 level lower than the T1 level of the dopant;        a LUMO level close to the workfunction of the cathode;        a HOMO level close to the workfunction of the anode;        high inter- and intra-chain order;        some degree of conjugation.        